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Filter Test Kits for 3D-printed Reusable Masks
An experimental apparatus has been designed for studying various filter materials and sizes in reusable respirators and masks. This will allow us to determine the optimal balance between the filtering performance and the breathing comfort of the user.COVID19
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The BEmask is still evolving toward meeting the requirements for a medical N95 mask. Functional design barriers to overcome include (1) providing an airtight seal around the face and nose for a large range of users, (2) user comfort with respect to the weight and bulk in comparison to disposable masks and (3) breathing ease for available filter materials that will meet N95 requirements.
The BE masks are being tested in 3 sizes (S, M, L) in either rigid or flexible materials. PLA is a rigid material that is easy to print and the user can be easily custom fit the nose area by thermoforming with hot water. TPU is compliant and forms a softer interface with the skin of the user, but it is more tricky to print.
Comfort regarding the weight and bulk has been addressed by using a 3D printed variation of a head strap arrangement that was used by 3M for their bulky industrial respirators (see https://www.youmagine.com/designs/comfortable-head-strap-for-reusable-respirators-and-medical-masks#information).
Our filter test kits are special variations of the BEmasks to study the effects of different filter materials, their thickness and their surface area on the ease of breathing in a 3D printed mask.
Specifically, we have found that for commercially available HEPA filters with a MERV rating of 15 (found in some vacuum cleaner bags), two layers of filter may be required to meet the N95 standards. We would like to know how large such filters must be in order for the user to breathe with an acceptable level of comfort.
To study this, we have designed a filter holding box that can adjust the filter area by masking off a portion with the blue tape (familiar to users of 3D printers). The adapter has two filter compartments for a standard (2 1/2 inch) filter size. Therefore, the user can evaluate the breathing comfort with up to double the filter size for single-layer and for double-layer filters.
The 3 mask sizes (S, M, L) have been modified so that they easily accept the filter holding box. The evolution of the design can be followed on our website https://bemask.org/.
Materials and methods
Parts can be printed in PLA and/or TPU without support. However, the Test Box Top has built-in supports that can be easily removed with the fingers. Parts will fit together with a better seal if connecting parts alternate between compliant TPU and rigid PLA. However, the kit works if printed in all PLA. The box can be sealed with tape if there is doubt about the seal.
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Turns out that to meet requirements for protection against aerosol transmissions, the BEmask design needs 2 layers of MERV 15 filter material which makes breathing too difficult to be practical, and the seal to the face must be perfect also.
My opinion (from our testing) is that the filter needs to be at least 8 square inches, double the original BEmask (or the Montana Mask) size. Testing for the efficacy against aerosol COVID-19 virus requires vigorous testing protocols that are not available to do-it-yourself designers. However, I am moving forward with 8 square inches (52 square centimeters) as a rule of thumb. Larger than that will be better than that for breathability at N95 standards.
of course this kind of apparatus can help human a lot and it can be so beneficial for them.